Electroactive polymers continue to attract attention due to their promising applications as functional materials for light-emitting diodes, electrochromic devices, field effect transistors, photovoltaic devices, batteries, antistats etc. One important class of inherently conductive or electroactive polymers are electrochromic polymers.
Electrochromic devices are well known, e.g., U.S. Pat. Nos. 4,902,108 and 6,178,034, incorporated herein in their entirety by reference. Such devices undergo a change in electromagnetic radiation transmission upon application of an electrical stimulus and have found use in a number of commercial applications. For example, they may be employed in glazings, e.g., energy efficient and privacy windows for architectural or automotive use, automotive rearview mirrors, displays, filters, eyewear including goggles, antidazzle and fog penetrating devices, and other applications where variable light transmission is desired.
Electrochromic devices are typically associated with a noticeable change in color. Changes in other optical properties, such as in the degree of clarity and opacity and absorption in the IR, are also characteristics of such devices.
In electrochromic materials, electrochemical oxidation or reduction induces a reversible change in the reflected or transmitted light. Electrochromic materials have proved especially useful in the construction of mirrors, displays, windows etc where the transparency or color of the article is altered by applying or altering an electrical voltage. Commercial devices include rear view mirrors that darken at night to prevent glare from headlights, or windows that darken to reduce transmitted sunlight or to provide privacy.
Many electrochromic devices have been produced using inorganic compounds like tungsten trioxide and iridium dioxide, but organic compounds, such as organic conducting polymers, continue to find increasing use as electrochromic materials. Among the advantages organic materials offer is that organic materials can more easily be fashioned into flexible devices such as would be used in electronic paper or other such applications.
Electrochromic materials include compounds that change from one color to another with applied voltage as well as compounds that change from transparent or clear to opaque or colored. The change from clear to colored can occur when a material is electrochemically oxidized, anodically coloring, or when the material is electrochemically reduced, cathodically coloring. The reverse reaction, for example back to clear, should occur when the electrical impulse is removed or reversed.
The fact that a cathodically coloring material changes from colorless to colored, or lightly colored to darkly colored, when reduced, does not mean that the material is colorless when in the neutral state. In many cases, such as with many thiophene polymers, the materials are clear when in a stable oxidized state, e.g., a cationic state formed in the presence of a polysulfonic acid, and becomes colored when the cationic material is reduced to the neutral state. More than one color may be formed depending on the applied voltage and “colorless” is a relative term, some small amount of color may be present in the colorless state with many electrochromic materials.
U.S. Pat. No. 6,791,738, incorporated herein in its entirety by reference, provides electrochromic polymers and devices. In particular, anodically coloring polymers having a band gap>3 eV in the neutral state and oxidation potential<0.5 vs a saturated calomel electrode, such as poly 3,4-dialkoxypyrroles, are provided.
Unless otherwise specified, when used herein “polymers” is a term including both co-polymers and homo-polymers.
U.S. Pat. No. 4,697,000 disclose the production of electronically conductive polypyrroles, which may be co-polymers of variously substituted pyrrole repeating units.
U.S. Pat. No. 5,446,577, incorporated herein in its entirety by reference, discloses display devices comprising a transparent outer layer, a first electrode, which is ion-permeable, having a reflective surface facing the transparent layer, an electrochromic material, preferably a conductive polymer such, as polyaniline, located between the reflective surface and the outer transparent layer, an electrolyte in contact with the electrochromic material and a second electrode located behind the first electrode. The display devices are capable of changing reflectance and/or color by the application of an electric potential to the electrodes.
U.S. Pat. No. 5,995,273, incorporated herein in its entirety by reference, discloses an electrochromic display device having an electrochromic conducting polymer layer in contact with a flexible outer layer; a conductive reflective layer disposed between the electrochromic conducting polymer and a substrate layer; and a liquid or solid electrolyte contacting the conductive reflective layer and a counter electrode in the device.
Inherently conductive materials based on organic polymers offer many advantages over metal or other inorganic materials in electrochromic devices. For example, polymers are often more readily processed providing improvements in device construction. Many conductive polymers are handled easily in air and can be molded or processed using conventional techniques well known in plastic and coating applications. Soluble polymers can be applied as a coating or via an ink jet or standard lithography process.
However, there are also potential disadvantages in using conductive polymers. Often, the polymer must remain in contact with an electrode or other surface. As with inks and coatings, adhesion to the surface must be attained and retained. Poor contact with, for example, an electrode, or subsequent delaminating will negatively impact or negate the desired electrochromic behavior. Further, many electrochromic applications place electrochromic polymer in the presence of electrolyte systems which may include aggressive solvents. The same solubility characteristics that allow a polymer to be applied as a coating may also result in a greater degree of polymer dissolution or delamination.
Many anodically coloring polymers face an additional problem in that the intensity or color strength of the colors that are formed upon voltage application or variation is not particularly strong, particularly when compared with the colors produced using cathodically coloring polymers. Copending application U.S. 61/125,689 incorporated herein in its entirety by reference, discloses an anodically coloring, electrochromic composition with improved color characteristics and durability, comprising an electrically active, anodically coloring polymer and a non-electrically conductive polymer. Surprisingly, it was found that the color of anodically color polymer can be enhanced by blending with non-conducting polymers.
Many cathodically coloring polymers are much more strongly coloring than anodically coloring polymers but still face significant challenges. For example, polymers that are easily processed because of good solubility are often more likely to delaminate from the electrode during use. Physical or chemical degradation may also occur resulting in reduced response to applied voltage seen in slower switching times, lower contrast ratios and device failure.
Any attempt to improve the performance of electrochromic polymers, for example, the adhesion or durability of the polymer, must not impact the characteristics of the polymer that make it valuable. For example, fast switching times, color space and sharp differentiation between the electrically oxidized or reduced forms of electrochromic materials can not suffer as a consequence of improved electrode adhesion or polymer durability.
It has been found that blending electrochromic, cathodically coloring polymers with non-electrically conducting binder polymers not only improves film forming properties, but also enhances performance after repeated switching, maintaining excellent switching times and color characteristics.